There has been proposed a process for producing information recording discs of an air sandwich structure made of plastics to perform storing and reading of information by converging rays of light such as laser beam and the like on recording medium containing layers, which process comprises subjecting two disc substrates through outer and inner peripheral spacers lying therebetween to ultrasonic welding (e.g. Japanese Patent Laid-Open Publn. No. 103537/1985).
The proposed process is intended to laminate two disc substrates to each other, wherein outer and inner peripheral spacers both having projections on both surfaces thereof are laid between said two disc substrates, ultrasonic wave vibration emanated from a ultrasonic welding machine is applied through said two disc substrates to said outer and inner peripheral spacers, and the projections of said outer and inner peripheral spacers are softened, thereby fusion bonding said two spacers to said disc substrates at once, and this process has such advantages that the process is simple and rate of production is high in comparision with the adhesive lamination method.
In the prior art process as referred to above, however, the ultrasonic welding machine used therefor is so designed as to leave spaces equal in distance between the horn and anvil in the positions of the outer and inner peripheral spacers at which the ultrasonic welding is performed, while the inner and outer peripheral portions of the disc substrate are not equal in thickness but generally the inner peripheral portion of the disc substrate is thicker than the outer peripheral portion thereof and, moreover, the outer and inner peripheral spacers are not equal in thickness. On that account, there were involved such problems that when such disc substrates having interposed therebetween such outer and inner peripheral spacers are ultrasonic welded together using the horn and anvil having a uniform space in distance therebetween, warp occurs on the substrate surface and no information recording discs having an angle of warp in conformity to the standard specification (within 5 mrad according to ISO) are obtained and that the welded portions are liable to peel off.